On December 5th, 1952, a veil of fog rolled over the city of London. It was the start of the deadliest air pollution disaster in British history, and more than sixty years later, an international team of chemists has figured out why.

The London Fog, one of the attractions of London in the early 20th century, was actually a deadly smog. It covered the city for 90 days a year. Here are incredible images of this beautiful but menacing haze.